At present gallium is mainly produced in processing high-quality ores such as bauxites.
Bauxites are practically not processed with the only purpose of extracting gallium. It is produced concurrently from intermediate products in which it is concentrated in processing beuxites to produce alumina.
In the Bayer processes for the treatment of beuxites 75% of gallium passes into aluminate solution as sodium gallate.
As a result of incomplete decomposition of bauxite slimes formed after leaching also contain gallium (about 30% of its content in the initial ore). When aluminate solutions are further processed gallium is again distributed between the solution and the aluminum hydroxide precipitate. The accumulation of gallium in the solution results in an increase of concentration of gallate which, in turn, increases its coprecipitation with aluminum.
After separation of the bulk of aluminum the concentration of gallium in the solution is 0.15 to 0.5 g/l.
Thus, it is the aluminate solution that is industrially important for producing gallium from bauxites.
The production of gallium from aluminate solutions takes two directions:
production of gallium concentrate, and PA1 direct extraction of gallium from said aluminate solutions.
It has been practically established that the content of gallium in the aluminum hydroxide precipitate depends on its content in the solution. Therefore, a certain alteration in the process of precipitation of aluminum hydroxide from the alkaline-aluminate solution may considerably reduce the amount of gallium in aluminum hydroxide. Gallium precipitates at the end of the process. A mixture of hydroxides produced by this method contains 0.2 to 3% of gallium by mass and is a raw material for its extraction.
In the known method the reusable aluminate solution of the Bayer process is treated with lime in autoclaves for precipitation of most of aluminum as calcium aluminate. After separation of the precipitate by filtration under pressure the solution is treated with carbon dioxide to produce a precipitate containing 0.3 to 1% of gallium by mass (see Acta Chemic., Sci.Acad.Hung., 1956, No. 14, p.1).
In another known method the bulk of aluminum, up to 90% by mass, is precipitated by carbonization, i.e. by treatment of the aluminate-alkaline solution with carbon dioxide, with stirring, the precipitate of hydrate of aluminum oxide is separated, and the remaining solution, containing caustic alkali, is subjected to recarbonization to produce a solution containing bicarbonate alkali, and a gallium-containing precipitate, comprising, in percent by mass: gallium oxide, 0.45; aluminum oxide, 47.4; sodium oxide, 18.4; carbon dioxide, 23.6; and water, 9.5 (see U.S. Pat. No 2,574,008).
After dissolving the gallium-containing precipitate with alkali, gallium is extracted by electrolysis from the aluminate-gallate solution produced. Electrolysis is carried out in stainless steel baths with stainless steel cathodes and anodes. The process is conducted at 3 to 4 V depending on the composition of the solution.
In a third method gallium is reduced from the aluminate-gallate solution by electrolysis on mercury or sodium amalgam (see French Pat. No. 1,261,344, and Journal of Metals, 1956, No. 8, p.1528).
As distinct from bauxites, nephelines contain a considerable amount of alkalis in the form of alkali metal oxides.
The necessity of removing alkali from the processing of nephelines reduces the frequency of circulation of solutions in the technological cycle of production of alumina, and the content of gallium in the aluminate-alkali solutions does not exceed 0.02 to 0.03 g/l.
Extraction of gallium from such solutions by electrochemical reduction is practically impossible, and the known methods of concentration do not make it possible to produce gallium concentrate from which it would be economically profitable to extract gallium.
The known methods for processing such low-quality aluminum-containing ores as clays, kaolins, alunites, and slates do not provide prerequisites for using them for concentration of gallium in processing nephelines either.
Thus, a review of the known methods for processing aluminum-containing ores and methods for concentration of gallium in them which make it profitable to extract gallium from intermediate products in the production of alumina, shows that it is impossible to employ the known methods for the production of gallium in processing nephalines.
It is an object of the invention to provide such a method wherein it is possible, at a comparatively low cost, to extract gallium from aluminate-alkaline solutions in the processing of high-silicon aluminum-containing ores such as nephelines.